a) Field of the Invention
The invention is directed to a plasma radiation source which emits radiation proceeding from a source region in a vacuum chamber along an axis of the mean direction of propagation of the radiation at a defined solid angle through a gas curtain that is provided for debris suppression.
b) Description of the Related Art
The invention is further directed to an arrangement for generating a gas curtain as a filter for particles in radiation whose mean propagation direction in a vacuum chamber extends along an axis directed through the gas curtain.
Plasma radiation sources are used for generating short-wavelength electromagnetic radiation (λ<110 nm). Lithography using extreme ultraviolet (EUV, 5 nm<λ<50 nm) is mentioned by way of example. The efficiency of plasma radiation sources depends upon the amount of radiation emitted in the desired wavelength interval in the usable solid angle and upon the size of the usable solid angle. The efficiency of the conversion of energy supplied to the plasma into usable radiation of the desired wavelength interval is known as conversion efficiency (CE). This depends on the plasma conditions (pressure, temperature, density, confinement time, material composition) as well as on the usable solid angle. The plasma can be generated either by an electric gas discharge or by particle bombardment or can be excited by intensive laser radiation.
In order to make use of the generated radiation, optical components are often needed for beam shaping. For lack of sufficiently transparent materials in the above-mentioned wavelength range, these optical components are mirrors or diffraction optics of varying complexity. For the same reason, it is necessary to evacuate the beam guiding system to a pressure that is sufficiently low to prevent gas absorption. Because of this, the optics are directly exposed to the damaging influence of the plasma or debris as it is called. By debris is meant fast particles that escape from the plasma and evaporated or sputtered material from the surroundings of the plasma. This influence limits the lifetime of the required optics. Suitable steps must be taken to ensure a sufficient lifetime of the optics. In principle, this can be accomplished in two different ways:
First, the plasma radiation source can be constructed in such a way that it generates as little debris as possible. This can be influenced, for example, by the type of plasma excitation (gas discharge, particle bombardment, laser excitation), by the design of the plasma environment, and by the choice of material composition of the plasma. Steps of this kind generate additional boundary conditions which frequently work against an optimization of the conversion efficiency. For example, xenon plasmas are often generated in plasma radiation sources for EUV lithography because xenon, as a noble gas, does not undergo any chemical changes and is not precipitated on surfaces. However, xenon is suboptimal from the view point of conversion efficiency in the wavelength range required for EUV lithography. Tin and lithium would be more favorable, but have rarely been used heretofore due to their low melting point and the high debris burden associated with it.
Second, active steps can be undertaken to protect the optics as far as possible from the damaging influence of debris.
Previously known active steps for suppression of debris involve deflection by electric and/or magnetic fields (e.g., U.S. Pat. No. 5,991,360), adsorption on surfaces in the form of a rotating foil structure, or foil traps and mechanical shutters.
While the latter are limited to only very small aperture surfaces in short-pulsed plasma radiation sources because fast shutter times could not be realized otherwise, electric and/or magnetic fields do not act on uncharged or quasi-neutral particle ensembles.
Rotating foil structures present problems with respect to suitability for storage under vacuum, mechanical stability over occurring centrifugal forces and high-precision balancing due to the high rotational speeds on the order of 1000 m/s that are required for intercepting very fast particles.
Foil traps comprise (metal) foils having gaps to which a flow of gas is applied and which must be arranged exactly longitudinal to the radiating direction to prevent power loss. Because of alignment tolerances, manufacturing tolerances and thermal loading, this requirement is often not met with sufficient precision in practice. On the other hand, a sufficient suppression of debris is only achieved with high gas flow rates. This leads to problems in the vacuum system and to loss of radiation output through gas absorption.
Similar problems can also occur in a device for x-ray irradiation according to EP 0 174 877 B1 which provides a flat flowing gas layer in front of an exit window of an evacuated chamber to protect a mask from particles.